As is well known, a clutch friction wheel or clutch plate, and the various components which are fixed to it, are sometimes subjected in operation to disagreeable vibrations, constituting what is often called judder. This judder effect is just as likely to occur at low torques, such as during a parking maneuver, as it is at high torques such as during a gear change while travelling or during a hill start.
The support surface zones of the liner support disc sometimes have a larger surface area than is required, and this gives rise to a phenomenon of interference, sometimes called embedding, in which the friction liners become embedded in the hollows that exist between two support surfaces of one blade of the support disc. This reduces flexibility, which is at least partly responsible for judder effects at maximum load. This lack of flexibility can occur very suddently.
In order to overcome this drawback, it has been proposed in the specification of French published patent application FR 2 094 693A to increase as far as possible the number of support surfaces provided by each of the blades, and also to increase the number of the blades themselves. The drawings of that document show the blades as having an asymmetrical shape, these blades being formed with cut-out portions. As a result, the blades do not have as great a resistance to centrifugal force as might be desired.
In addition, there is a problem of contact with the pressure plate of the clutch. In each operation of engaging the clutch, the friction liners become progressively gripped between the pressure plate and the reaction plate of the clutch. During this operation considerable frictional forces are set up, which give rise to heating. This heating leads inevitably to a gradual deformation of the pressure plate into a conical shape, as is explained for example in U.S. Pat. No. 2,902,130. The same effect is also observed in the reaction plate, though not to such a marked extent.
As a result, the zone in which the friction liners are subjected to the pressure between the liners and the plates of the clutch gradually shifts radially inwards, i.e. towards the axis of the assembly. This causes unequal wear of the friction liners to occur, and also reduces their efficiency by reducing the torque which is transmitted. It may also give rise to judder effects.
In general terms, and especially in the interests of standardisation, it is desirable that the liner support disc should be suitable not only for friction liners of the organic type, but also for those of other types.